Elastic search - facet on nested object

data.js

{
   "obj1":[
      {
         "date":"2012-01-01T00:02:00Z",
         "id":4
      },
      {
         "date":"2012-01-01T02:11:00Z",
         "id":6
      }
   ]
}

mapping.js

{
   "type1":{
      "properties":{
         "obj1":{
            "type":"nested",
            "properties":{
               "date":{
                  "type":"date",
                  "format":"dateOptionalTime",
                  "include_in_all":false
               },
               "id":{
                  "type":"long",
                  "include_in_all":false
               }
            }
         }
      }
   }
}

query.js

{
   "facets":{
      "facet1":{
         "date_histogram":{
            "field":"date",
            "interval":"hour"
         },
         "nested":"obj1"
      }
   }
}

result.js

{
  "took" : 1,
  "timed_out" : false,
  "_shards" : {
    "total" : 5,
    "successful" : 5,
    "failed" : 0
  },
  "hits" : {
    "total" : 1,
    "max_score" : 1.0,
    "hits" : [ {
      "_index" : "test",
      "_type" : "type1",
      "_id" : "1",
      "_score" : 1.0, "_source" : {"obj1" : [{"date" : "2012-01-01T00:02:00Z","id" : 4},{"date" : "2012-01-01T02:11:00Z","id" : 6}]}
    } ]
  },
  "facets" : {
    "facet1" : {
      "_type" : "date_histogram",
      "entries" : [ ]
    }
  }
}